accination is the most cost-effective means of preventing respiratory syncytial virus RSV) disease. A feasible approach to a safe RSV vaccine is to make a subunit of the virus, and the best candidates are the attachment glycoprotein (gp90) and/or the fusion glycoprotein (gp70). The fusion (F) protein is able to elicit neutralizing and antifusion antibodies, and has been shown to protect against RSV infection in animal models. Purification of the fusion protein from is natural source is difficult and expensive for its commercial application. Overproduction of a recombinant form of the F protein in E. coli will facilitate its use as a vaccine. However, expression of the entire F protein is toxic to the bacterial cell, hampering overproduction. These studies will take truncated cDNA clones of the F protein and attempt to overexpress an antigenic F product in E. coli using high-level expression vectors. Immunogenicity of the recombinant protein will be examined using bothe neutralization and anti-fusion assays. Phase II studies will be the development of procedures amenable to large-scale purification of the cloned protein and protection studies with additional testing of immunogenicity.